They also added the most new anatomical features.

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We tend to think of feathers as one of the defining features of birds. But in recent years, it's become apparent that the lineage of dinosaurs that gave rise to birds (the Theropods) had feathers millions of years before anything remotely bird-like existed. Just last week, feathers were also found on a dinosaur outside the theropod lineage, raising questions about what made the theropods special (aside from, well, all sorts of cool dinosaur species).

Previous attempts to detect any global trends in bird-like traits among the theropods haven't come up with anything definitive. But today, scientists are releasing a new computer analysis of thousands of traits from theropod dinosaur fossils. The results show that the lineage that gave rise to birds has been getting smaller for 50 million years, and it underwent a huge burst of adding novel anatomical features. Both of these revelations are in sharp contrast to the rest of the theropod lineage.

The first theropods appear in the fossil record after the Triassic-Jurassic mass extinction event. During the early part of their history, they were notable for being rather large and static. Some groups appeared in the fossil record 180 million years ago, persisting right up to the mass extinction event that ended the non-avian dinosaurs. And one of the earliest groups to split off the lineage that led to birds were the Megalosaurids—which, as their name implies, were rather large.

A number of studies of theropod fossils have looked at whether there were any trends within the lineage that would provide some hint that birds were in their future. But feathers appear to have been widespread, and there was no lineage-wide trend toward a smaller size.

So to get a clearer view, an international team of researchers used two massive databases of fossils and their characteristics. One data set covered 120 different taxa within the theropods and had measurements of 1,549 different skeletal characteristics. The second was smaller (100 taxa, 421 characters) but had each feature re-analyzed to ensure that all the data was accurate.

The researchers then fed all this data to a piece of evolutionary analysis software called BEAST (Bayesian evolutionary analysis sampling tree). BEAST was originally designed to build family trees for viruses and make inferences about what their extinct common ancestors looked like. With appropriate modifications, it is now able to help researchers learn the same about the common ancestors of animals, based on physical features rather than DNA sequences.

The results show that about 200 million years ago, animals near the base of the avian lineage were probably in the area of 160 kg. These weren't committed to shrinking, however, as they're the ones that gave rise to the Megalosaurids. By 25 million years later, that ancestor had shrunk to 46 kg, but it was still giving rise to lineages like the Allosaurs and Tyrannosaurs. Ten million years and many branches (including the raptors) later, the last group to split off from the birds probably weighed only 3 kg, while the first members of the bird lineage were less than a kilogram.

Both data sets show this general downward trend in size, followed by a sharp drop during a roughly 20 million year period. But the theropods as a whole showed no trend in any direction during this time, so a static picture of the lineage wouldn't have revealed this shrinkage.

That 20 million year period also corresponds to a rapid diversification of the theropods, starting with the origin of the Allosaurs and Tyrannosaurs. But at least seven other major groups split off from the avian lineage during that period, and the authors show that this diversification was associated with rapid change in the skeletons of these animals. "Every branch along the avian stem evolves at least twice as fast as the average theropod 'background' rate," the authors write. They refer to this period as seeing the "near simultaneous stratigraphic appearance" of groups including Allosauroidea, Tyrannosauroidea, Compsognathidae, Alvarezsauroidea, and Paraves.

The authors suggest that it was the small size of these animals that allowed them to undergo rapid shifts in body plan, like the reorganization of their distribution of weight and greater agility. All of that might have been difficult to manage in something as large as a tyrannosaur.

The picture the work paints is of a lineage that was able to diversify rapidly and sample a huge range of body forms and sizes, from the enormous tyrannosaurs to small climbers that weighed less than a kilogram. Although it wasn't inevitable that this would eventually give rise to the only surviving dinosaurs, it indicates that many of the pieces were put in place millions of years before anything avian appeared in the fossil record.